Herbicide Transport in a Managed Riparian Forest Buffer System

The effect of a riparian forest buffer system (RFBS) on transport of two herbicides, atrazine and alachlor, wasstudied during 1992-1994. Herbicides were applied to an upland corn crop in March of each year. The buffer system wasmanaged based on USDA recommendations and averaged 50 m in width. The system included a grass buffer stripimmediately adjacent to the field (Zone 3); a managed pine forest downslope from the grass buffer (Zone 2); and a narrowhardwood forest containing the stream channel system (Zone 1). After the first year of the study, the managed forest wasclear-cut in 1/3 and thinned in 1/3 of Zone 2. The other 1/3 of Zone 2 was left as mature forest. Most of the herbicidetransport in surface runoff occurred before 30 June with about 25 cm of cumulative rainfall after herbicide application.During this period of higher herbicide transport, atrazine and alachlor concentrations averaging 34.1 g L1 and 9.1 g L1at the field edge, respectively, were reduced to 1 g L1 or less as runoff neared the stream. There were generally nodifferences among the mature forest and the two treatment areas (clear-cut and thinned) for either concentration or load insurface runoff. Using precipitation data collected on site, the effects of dilution versus other concentration reduction factors(infiltration, adsorption) was estimated for surface runoff. Concentration reduction was greatest per meter of flow length inthe grass buffer adjacent to the field. There was only minor transport of herbicides through the buffer system in shallowgroundwater and little difference between the Zone 2 treatment areas. In 1992 and 1993, herbicide concentrations in shallowgroundwater in the RFBS and at the edge-of-field were generally at or below detection limits. In 1994, well concentrations ofboth herbicides increased, probably in response to infiltration of surface runoff containing high herbicide concentrations.Average herbicide concentrations were at or below detection limits in groundwater near the stream for most of 1994.